Sexual Reproduction

Sexual Reproduction

1. During intercourse the penis of a sexually excited male becomes erect as blood fills the spongy erectile tissue, the penis is inserted into the vagina and its movement result in ejaculation of semen into the vagina. The semen travels along the urethra and the force of the ejaculation is enough to propel some through the cervix and into the uterus, with the remainder being deposited at the top of the vagina. The sperm swim through the uterus into the fallopian tubes by the lashing movements of their tails. However only a few hundred sperm reach the site of fertilisation and surround that oviduct.

2. Fallopian Tubes: The lumen of the fallopian tubes is lined with ciliated epithelium, which move in a wave like fashion to propel the ova along. Fertilisation always takes place in the fallopian tubes.

3. The Vagina and Cervix: The vagina is a muscular tube into which the penis is inserted during sexual intercourse. The opening between the vagina and the uterus is nearly closed by a ring of muscle called the cervix.

4. Fertilisation and The Acrosome Reaction

   • Internal fertilisation ensures that the sperm are deposited in the females reproductive tract. From here the sperm need to go through the process of capacitation in order to be able to swim which can take several hours, this activation occurs by a change to the membrane surrounding the acrosome. They can then use their tails to swim through the cervix into the uterus and into the fallopian tube. The sperm can remain viable for 12-48 hours.
   • Once in the fallopian tube several hundred are needed to penetrate the ovum. The secondary oocyte is surrounded by the follicle cells and a clear membrane called the zona pellucida. As the egg can only survive for 24 hours once released from the Graafian follicle, this will only happen if ovulation has recently taken place.
     • As the head of the sperm binds to the cells of the zona pellucida, the acrosome reaction is stimulated. The acrosome membrane ruptures, releasing protease enzymes which act on the glycoproteins of the zona pellucida to soften them and allow the sperm to penetrate through. The head of the sperm also produces a protein filament which pierces the already softened portion of the membrane., enabling the sperm to penetrate the egg.
       • Once the sperm binds to the membrane it triggers a cortical reaction in the surrounding proteins that forms an impenetrable barrier to all other sperm called the 'fertilisation membrane'. This means that only one sperm is able to enter the ovum. Entry of the sperm also simulates the completion of the second meiotic division. of the oocyte nucleus. The nuclei of the sperm and egg are drawn together to form a diploid nucleus.

5. Implantation: After fertilisation the zygote begins to divide by mitosis to form a ball of cells called the blastocyst. It embeds into the endometrium after about three days once it has moved from the fallopian tube this is called implantation. The outer layer is called the tropho blast which develops into two membranes the chorion and the amnion. The chorion forms a number of finger like projections called the chorionic villi, these increase the surface area for absorption of nutrients from the uterus. It also secretes human chorionic gonadotropin (hCG) which maintains the corpus luteum, the detection of this in urine is the basis of most pregnancy tests. The chorionic villi eventually form part of the placenta, which is attached to the foetus by the umbilical cord.

6. Sub-fertility and Infertility

   • #Sub-fertility: Difficulty in conceiving naturally for reasons affecting the male, female or both partners.
   • #Infertility: The complete inability to conceive a child.
     • Very rare
   • Two main causes
     • Failure to ovulate, usually associated with the absence of or an irregular menstrual cycle. Treatable in 95% of cases with a drug called domiphene.
     • A blockage of the fallopian tubes, preventing the passage of the ovum to the site of fertilisation in the fallopian tubes. May be caused by infection and treatment usually involves microsurgery.

7. Pregnancy Testing

   • #Monoclonal Antibodies: Many types of one type of antibody
   • #Antigen: A non-self-protein that is recognised by the immune system and brings about an immune response
   • Most Pregnancy testing kits use monoclonal antibodies to test for the hormone hCG, in urine. A monoclonal antibody is one that responds to only one type of foreign antigen, the one used in pregnancy testing kits is specific to hCG which acts as an antigen. hCG is excreted n the urine and high levels results as a positive pregnancy test. The test relies on the reaction between antibodies bound to coloured latex beads and hCG, with a second row of antibodies to produce a colour change.

8. In Vitro Fertilisation (IVF)

   • A couple are described as infertile if they have failed to conceive a child after 12 months.
   • A couple seeking fertility treatment will first be assessed to try to discover the cause of the problem. In the UK these can be divided into three: 50% are problems with the females reproductive tract. 35% are related to the male. 15% cannot be explained
   • Process: Ovulation is stimulated using hormones at a specific dosage which aims to cause several follicles to develop at the same time. A guided by ultrasound is inserted through the vagina and into the fallopian tubes to collect the oocytes. On the same day the males semen is collected and placed in a liquid containing nutrients. Each oocytes placed In a separate dish and about 100 000 sperm are added to each. (An alternative method involves injecting the sperm nucleus into the oocyte.).
     • Three days later the oocytes are examined to see which have been fertilised. Two are inserted into the uterus using a tube, to increase the chance that one will implant.